1. Technical Field
The present invention relates to a purification catalyst for exhaust gas, to a production method therefor, and to a purification catalyst device for exhaust gas, and specifically relates to a production technique for a purification catalyst for exhaust gas in which nitrogen oxides (NOx), hydrocarbon (HC), and carbon monoxide (CO) contained in exhaust gas emitted from an internal combustion engine (for example, in a vehicle) can be simultaneously and effectively reduced, thereby reducing the undesirable components of the exhaust gas.
2. Background Art
For purifying exhaust gas containing, for example, CO, HC, NO, and NO2, it is well known that noble metal elements (Pt, Rh, Pd, Ir) exhibit high performances. Therefore, it is preferable to employ the above-mentioned noble metal elements in the purification catalyst for exhaust gas. These noble metals are generally supported or mixed with Al2O3 which is a support having a high specific surface area, with additives such as La, Ce, or Nd. On the other hand, noble metals can be combined with various elements by making composite oxides, the composite oxides can exhibit various properties compared to the noble metal alone, and performances for purifying exhaust gas can be improved. Furthermore, it is known that a purification catalyst for exhaust gas having superior heat resistance can be obtained in mixed condition or solid-solution condition of Pd-based composite oxides and transition metal-based composite oxides.
As such a purification catalyst for exhaust gas, a heat-resistant catalyst in which a Pd-based composite oxide comprising at least one element selected from rare earth metals and alkali earth metals and a composite oxide comprising at least one element selected from transition metals are mutually dissolved or used in a mixture, is disclosed in Japanese Unexamined Patent Application Publication No. 10-277393. In this technique, since the purification catalyst for exhaust gas is composed of the Pd-based composite oxide, heat resistance can be improved based on stabilization of fine Pd particles. Furthermore, since sintering-proof property of the Pd-based composite oxide is obtained by mixing the Pd-based composite and transition metal-based composite oxide, heat resistance can be improved.
However, it is necessary to burn at more than 1000° C. to form the Pd-based composite oxide in the above-mentioned current technique. Therefore, crystallization proceeds and Pd dispersion is deteriorated, and as a result, superior purification property cannot be obtained. Therefore, a purification catalyst for exhaust gas having superior purification property in which deterioration of Pd dispersion by crystallization is prevented even if composite oxide having the same amount of noble metal as in a conventional technique is burned at a lower temperature, is required.